I was surprised that the discovery of an unknown Cellini bronze in the Royal Collection passed with so little notice. Perhaps more notice would have been taken if the article in the August issue of the Burlington Magazine, admirable and exciting though it was, had mentioned the fact that the bronze in question had turned up, unidentified, in a cupboard in Buckingham Palace.

The bronze figure of a satyr, just under 60 cm high, is one of a pair. Its partner has for almost two decades been at the Getty Museum in California, and before then was in a collection in Vienna. But before then, according to the royal records, it too was in the Royal Collection. There is a 19th-century photograph of it in an inventory at Windsor. Someone either stole it, or borrowed it, or bought it, or (a perfectly possible explanation) was given it by some past monarch: "Oh, you like that thing, do you? Have it, by all means. No, I insist..."

So the story so far is: one of the greatest goldsmiths and most celebrated sculptors of the Italian renaissance, Benvenuto Cellini, made two figures of satyrs, as part of a model to demonstrate to the King of France how he would transform the Porte Dorée, the Gilded Gate, at Fontainebleau. The satyrs would flank the gate, and above it there would be a lunette representing, at the king's request, the genius of Fontainebleau.

Only the lunette was made. It is one of the treasures of the Louvre, and it is enormous: it is four metres wide, and depicts the presiding nymph of the spring at Fontainebleau, caressing the head of a stag, which is the emblem of the king, François Premier [Francis the first]. Incorporated into the design are other animals - wild boar, deer and dogs including the hunting dog called Bleu or Bliaud, who was responsible for finding the spring (hence the name Fontainebleau).

Cellini has not always been considered a particularly great artist. His autobiography, first published in the 18th century, had great success in the 19th (the opera by Berlioz being its greatest by-product) but people read it as a terrific yarn by a thoroughly engaging but boastful and not very reliable source. In those days, Cellini's works, with the exception of the great bronze Perseus in Florence, were largely dispersed and unrecognised.

The famous salt-cellar (recently stolen from Vienna) sat in Schloss Ambrass, and was only identified on the basis of Cellini's description of it. The stunning bust of Bindo Altoviti, now in the Isabella Stewart Gardner Museum, was on the art market for years, marked down by the experts as a fake. The most celebrated drawing by Cellini, which depicts the satyr now at the Getty, came from the collection of Sir Thomas Lawrence, but remained unidentified until the 1960s, when a visitor to the gallery where it was hanging figured out that "fontana Bellio" in the inscription referred to Fontainebleau. (That drawing, now in the Washington National Gallery of Art, has become recognised as one of the great masterpieces in American collections.)

So it is not surprising that the Royal Family, in this particular instance, didn't know what they had, even if it was eccentric to keep such a thing in a cupboard. The Buckingham Palace bronze differs from the one at the Getty in that it has not been cleaned up and given a fancy patina at any point. But it has this sad feature in common with its Getty brother: both figures were emasculated (had their genitalia chopped off, is what I mean) and fig-leafed, perhaps in the 18th century.

Comparing the two photographs (I have not had the opportunity to see either bronze in real life) you can see how the history of a piece of sculpture radically affects its appearance. It's like one of those studies of identical twins separated at birth, excepting that these two figures would only have been separated perhaps a century ago. Nobody apparently knows.

Cellini describes the figures he made: "I fashioned two satyrs, one upon each side [of the gate]. The first of these [the one in the Getty] was in somewhat more than half-relief, lifting one hand to support the cornice, and holding a thick club in the other; his face was fiery and menacing, instilling fear into the beholders. The other [the one in the Buck House cupboard] had the same posture of support; but I varied his features and some other details; in his hands, for instance, he held a lash with three balls attached to chains." When the king saw the whole model, it "restored him to cheerfulness, and distracted his mind from the fatiguing debates he had been holding during the past two hours".

And may we hope that when Her Majesty saw the remaining figure, it restored her to cheerfulness as well, if she needed restoring? Had she taken it onto "Going for a Song," she would inevitably have been asked whether she had it insured, and if she had the remotest idea of what its value at auction might be. And then the expert would have mentioned, I suppose, a seven or eight-figure sum - respectfully adding, of course, that the bronzes would have been much more valuable as a pair.